1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus for use on an automobile.
2. Description of the Related Art
An electrically operated power steering apparatus on automobiles include a rack shaft which is axially movable by a pinion coupled to a steering shaft with a steering wheel mounted on an upper end thereof. When the rack shaft is axially moved by the pinion upon rotation of the steering wheel, tie rods connected respectively to opposite ends of the rack shaft are moved to turn or steer respective road wheels. At this time, the steering torque of the steering shaft is detected, and an electric motor combined with the rack shaft is energized based on the detected steering torque. The force produced by the electric motor is transmitted through a ball-and-nut mechanism to the rack shaft to assist the rack shaft in its axial movement for steering the road wheels. One such electrically operated power steering apparatus is disclosed in Japanese laid-open patent publication No. 62-261573 which corresponds to U.S. Pat. No. 4,666,014.
The disclosed electrically operated power steering apparatus includes a pinion housing which accommodates a pinion rotatable with a steering shaft, a motor housing which accommodates an electric motor, and an outer housing which accommodates a ball-and-nut mechanism for transmitting the force generated by the electric motor to a rack shaft. The pinion housing, the motor housing, and the outer housing are fastened into a housing assembly.
Since the housing assembly is composed of three independent housings, the number of parts used and the number of fastened regions are relatively large, the strength with which the rack shaft is supported by the housing assembly is relatively low, and it is tedious and time-consuming to assemble the housing assembly. The housing assembly is assembled by superimposing opposite ends of the motor housing on respective ends of the pinion and outer housings, and fastening the superimposed ends with bolts. Therefore, the motor housing is larger in diameter than the pinion and outer housings, and the heads of the bolts project radially outwardly from the outer surface of the motor housing.
The electric motor is disposed around the rack shaft in the motor housing coaxially with the rack. The electric motor is supplied with an electric current through wires that extend from an external power supply into the motor housing in which the wires are connected to the electric motor.
Inasmuch as the electrically operated power steering apparatus has a high maximum current requirement and is installed in the engine compartment of an automobile, it is necessary that the wires have a large diameter with a highly heat-resistant, hard covering. It has been customary to use a rubber grommet as a seal between the motor housing and the wires.
The thick wires are introduced directly into the motor housing. Therefore, when the electrically operated power steering apparatus is assembled, it is cumbersome to position the wires neatly in place and fix the wires to the motor housing through the rubber grommet. The rubber grommet must be deep enough to provide a sufficient seal around the wires, but the deep rubber grommet requires that the motor housing be large in size.